Ethernet technology is simple, easy to use and cheap in price, and has been widely used in the range of local area networks, metropolitan area networks, and wide area networks. With the extensive use of Ethernet, the Operation, Administration and Maintenance (OAM) technique of the Ethernet has become increasingly important.
IEEE 802.1ag Connectivity Fault Management (CFM) defines the OAM functions and mechanisms used in the Ethernet service layer. ITU-T Y.1731 is a complement and enhancement to IEEE 802.1ag, complements the OAM function for error management and mainly enhances the OAM function for performance monitoring. In the IEEE 802.1ag draft, the network administrator, on the basis of the purpose of management and maintenance, divides the network into a number of nestable maintenance domains, a single maintenance domain being as shown in FIG. 1.
The maintenance domain in FIG. 1 define a series of maintenance points on edge devices and internal devices, in which hollow points represent Maintenance association End Points (MEPs), and solid points represent Maintenance domain Intermediate Points (MIPs). The OAM at the Ethernet service layer realizes the management and maintenance functions through the MEPs and MIPs.
The frame loss ratio measurement based on ITU-T Y.1731, which is carried out between two MEPs, is an important constituent part of Ethernet OAM technology and is an important means for performance monitoring. ITU-T Y.1731 defines two methods for measuring the frame loss ratio: single-ended frame loss ratio measurement (single-ended ETH-LM) and dual-ended frame loss ratio measurement (dual-ended ETH-LM). By periodically sending and receiving, between two MEPs, protocol frames loss measurement message (LMM) used for frame loss ratio measurement, loss measurement reply (LMR) and continuity check message (CCM)), two-way frame loss ratio measurements of a link are achieved. Each MEP maintains two counters as follows:
TxFCl: counter for in-profile data frames transmitted towards the peer MEP;
RxFCl: counter for in-profile data frames received from the peer MEP.
In the process of measurement, the MEPs at both ends will add values of the above-mentioned two counters into the protocol frame. The frame loss ratio measurement is carried out using the values of these counters.
The end initiating a frame loss ratio measurement is called the active end, and the other end is called the passive end. For single-ended frame loss ratio measurements, frame loss ratio measurements are carried out at the active end, which can measure the two-way frame loss ratio of a link according to the value of the data frame counter carried in the received LMR and the value of the local counter. For dual-ended frame loss ratio measurements, frame loss ratio measurements are carried out at both ends, with each end being able to measure the two-way frame loss ratio of the link according to the value of the data frame counter carried in the received CCM. Therefore, for dual-ended frame loss ratio measurements, both ends serve as the active end and the passive end at the same time, while for a certain measurement, it is still possible to call one end as the active end and the other end as the passive end.
The working principle of single-ended frame loss ratio measurements is as shown in FIG. 2, and the working principle of dual-ended frame loss ratio measurements is as shown in FIG. 3. The parts shown in italics in the figures represent the counter values newly added into the protocol frame during protocol frame interaction.
The formula given by ITU-T Y.1731 for calculating the single-ended frame loss is as shown in Formula 1, and the formula for calculating the dual-ended frame loss is as shown in Formula 2.Far end frame loss=|TxFCf[tc]−TxFCf[tp]|−|RxFCf[tc]−RxFCf[tp]|  (Formula 1);Near end frame loss=|TxFCb[tc]−TxFCb[tp]|−|RxFCl[tc]−RxFCl[tp]|  (Formula 1);Far end frame loss=|TxFCb[tc]−TxFCb[tp]|−|RxFCb[tc]−RxFCb[tp]|  (Formula 2);Near end frame loss=|TxFCf[tc]−TxFCf[tp]|−|RxFCl[tc]−RxFCl[tp]|  (Formula 2);
where [tc] represents the counter value in the currently received protocol frame and the local counter value at this moment, and [tp] represents the counter value in the protocol frame received at a previous measurement point and the local counter value at that moment.
However, the above-mentioned calculation process is set up on the basis that all the protocol frames used for measuring the frame loss ratio can be transmitted reliably, but an Ethernet link is not capable of providing such a guarantee. Once the situation where a protocol frame is lost during transmission over the link happens, it will cause errors in the flame loss ratio measurements.
As regards the problem in the related technique that frame loss ratio measurement is inaccurate in the case that a protocol frame is lost, no effective solution has yet come up.